Phased array antennae are used in communications systems and radar systems to provide adjustable directionality of transmission and reception, without the need to physically displace the antenna.
A phased array antenna consists of many individual antenna elements arranged in an array, typically a linear (one-dimensional) or matrix (two-dimensional) Layout. The elements are typically spaced from each other by a distance equal to one half of the wavelength of interest.
In reception mode, the signals received by each element are summed together to provide an overall received signal. With no phase difference introduced into the signals from the various elements, the antenna is most sensitive to signals arriving from a direction perpendicular to the plane of the antenna array. By introducing a progressive phase delay into the various elements, the direction of maximum sensitivity may be adjusted.
For example, FIG. 1 shows a one-dimensional array 10 of antenna elements 12-1 to 12-7 in which each element is associated with a phase delay 14-1 to 14-7 of a greater than that of the preceding element. A wave-front 20 is shown, arriving in the direction of maximum sensitivity. The path length 16-1 to 16-7 to be travelled by the wave-front increases from zero at element 12-1 to 6xcex2 at 16-7. The path length xcex2 is the distance travelled by wave-front 20 in the time taken for a phase angle of xcex1 to be travelled by wave-front 20. That is, xcex2=xcex.xcex1/2xcfx80, where xcex is the wavelength of the signal producing wave-front 20. The effect of this and the phase delays 14-1 to 14-7 is that summing unit 22 receives signals corresponding to wave-front 20 from each antenna element at the same time. These signals will sum 38 to produce a large response to wave-front 20. For signals arriving from different directions, such as wave front 30, the summing unit 22 will receive signals corresponding to wave-front 30 at different times, since the phase delay introduced by phase delay elements 14 is not compensated by a corresponding difference in path length. The signals received at the summing unit 22 will arrive at different times, and will not add up 38 to a large response. Signals corresponding to wave-front 34 will arrive over an even wider spread of timings, and the sensitivity of the array 10 to the wave-front 34 is even less than to wave-front 30.
The summer 22 may apply a weighting scheme to the various signals from antenna elements 12 to provide a degree of aperture shading.
By analogy, similar considerations may be applied to transmission of signals. That is, a single outgoing signal is applied to each of the phase delay elements 14-1 to 14-7. Due to the phase delays, the signal is transmitted from the various elements 12-7 to 12-1 with an increasing delay. The corresponding wave fronts produced by each of these elemental antennae will effectively sum to produce a wave-front principally orientated as in the direction of wave 20 shown in FIG. 1.
The above principles may be applied to a two-dimensional matrix array, whereby the directional sensitivity of the phased array antenna may be adjusted in both azimuth and elevation by suitably setting the various phase delays.
A great advantage of the phased array antenna is in that the directional response of the antenna may be altered electronically, by suitable control of electronic phase delay elements 14. The antenna may therefore be xe2x80x9cpointedxe2x80x9d in a required direction without any mechanical movement of the antenna. This allows for simplification of antenna installation, and allows the direction of the antenna to be changed very rapidly.
Another advantage of phased array antennae is in the improved signal/noise ratio in the final output signal. As the number n of elements in the array increases, the noise signal increases as n, while the signal strength increases as n. The improvement in signal-to-noise ratio, as compared to a single antenna element, is n. Accordingly, a high number of antenna elements should be used to give a good signal/noise ratio. Depending on the application, seven elements (as shown in FIG. 1) may provide sufficient directionality and signal-to-noise ratio. However, it is common to use much larger numbers of antenna elements. Some communications or radar receivers are known having tens of thousands of elements, each with their own associated phase delay. Such an arrangement obviously provides a much increased signal-to-noise ratio, but can lead to problems in processing such a large amount of data.
Since the antenna array can only receive or transmit one signal at a time, all of the signals from the numerous elements must be added together to produce a single output signal. As shown in FIG. 2, the antenna elements 12 may be arranged into sub-arrays 36. Each sub-array will contain the equipment illustrated in FIG. 1, that is, the associated antenna elements 12, their phase delay units 14, and a summing unit 22. Each sub-array 36 then produces a single output signal, 38, and these signals from the sub-arrays are summed in a further summing unit 40 to produce the required single output signal 42, representing a combination of the signals from all of the antenna elements. This arrangement avoids the need for a single summing unit to sum the possibly very numerous signals from the antenna elements 12. The use of sub-arrays also allows certain advanced types of signal processing to be carried out, such as blocking a jamming signal by adjusting the antenna response to ignore the jamming signal, as is known to those skilled in the art.
The phase delays xcex1, 14 applied to each antenna element are the same across all elements 12 in all subarrays.
Another known advantage of arranging the antenna elements into sub-arrays is in that the directional response of the antenna may be adjusted by adjusting the phase of the signals 38 produced by each sub-array.
As shown in FIG. 3, a different phase delay xcfx861-xcfx869 may be applied 48 to each of the sub-array signals 38. By treating each sub-array signal 38 in the same way as the element signals of FIG. 1, it can be seen that the response of the whole antenna may be adjusted by adjusting the phase delays xcfx861-xcfx869 48 appropriately. In a two-dimensional antenna such as that shown in FIGS. 2-3, the antenna response in both azimuth and elevation may be adjusted by adjusting the phase delays xcfx861-xcfx869 48.
Analogue-to-digital converters 44 may be introduced to convert the signal 38 from each sub-array 36 into a corresponding digital representation 46, allowing the phase delays xcfx861-xcfx869 to be introduced by digital phase shifters 48 before being summed by a digital summing unit 40.
While it is possible to adjust the directionality of the antenna by introducing appropriate phase delays into the sub-array signals 38, this is only possible within the response defined by the phase delays of the individual elements within the subarrays. For example, as shown in FIG. 4, the response of the antenna as defined by the phase delays of the individual elements in one dimension (azimuth or elevation) as already discussed with relation to FIG. 1, is shown as outer envelope 50 in this polar diagram. By introducing appropriate phase delays into the sub-array signals 38, the direction of maximum sensitivity may be controlled by introducing suitable phase delays xcfx861-xcfx869 48, but the range of this adjustment is limited by the overall response 50 of the antenna as defined by the phase delays 14 of the individual elements 12. For example, by adjusting phase delays xcfx861-xcfx869 48 to provide maximum response in directions 52 or 54, the sensitivity will be reduced by about half and significant side lobes will be produced. If phase delays xcfx861-xcfx869 were adjusted further to provide maximum response in a direction 56, for example, the antenna response would be very low.
Using this method, it is possible to provide an antenna response with a plurality of maxima. For example, the subarray outputs may be split a number of ways to form a number of simultaneous beams. By taking the signals output from the subarrays or the digital converters 44 and applying them to two or more sets of phase shifters 48 and summer 40, it is possible to have an antenna response in both directions 52 and 54. This may be achieved by applying different weighting factors to the various subarray signals, in the respective summers. Some subarray signals may have a weighting of zero applied, to eliminate those signals from derivation of the resultant antenna response. The weighting scheme may be variable or adaptive, according to requirements. Alternately, it is common to provide a sum beam for surveillance, and a difference beam for monopulse applications. This may be performed by using a same phase shift to each subarray signal, with differing weighting to provide sum and difference beams in a same direction, or the various phase shifts and weightings may be changed to provide simultaneous beams in differing directions.
A problem arises when performing beam steering and multiple beam formation by use of variable phase shifting and summing of subarray signals. As the phase centres of the subarrays are usually separated by many wavelengths, grating lobes arise.
An object of the present invention is to alleviate the limitations of sub-array beam steering by reducing the limitations on the range of beam steering that is possible by adjustment of sub-array signal phase delays.
Another object of this invention is to provide a method of operating a subarrayed phased array antenna to allow beam steering by control of sub-array signal phase delays and weighting of various subarray signals in an associated summing step over a wide range.
Accordingly, the present invention provides: a method for operating a phased array antenna, itself comprising a number of sub-arrays, comprising the steps of: introducing phase offsets into signals received from/transmitted to each element in the antenna, to provide directional transmit or receive sensitivity in a first direction; summing signals received from each element in each sub-array to provide respective sub-array signals; introducing respective phase offsets into the respective sub-array signals to adjust the direction of maximum sensitivity (beam magnitude) within a sensitivity envelope defined by the phase offsets applied to each element of the antenna as a whole; further comprising the step of: adjusting the phase offset of each element of each respective sub-array by a respective amount, thereby to alter the directional sensitivity of each respective sub-array, respective sub-arrays having the phase offset of their elements adjusted by differing amounts (xcfx861-xcfx869), thereby providing the sub-arrays with a variety of directional sensitivities diverging from the first direction.
Each sub-array may be controlled such that each signal produced by an antenna element is adjusted to direct the maximum sensitivity of the sub-array in a different direction from the maximum sensitivity of other sub-arrays.
Adjustment of the phase offsets applied to the sub-array signals may allow the maximum sensitivity of the whole phased array antenna to be adjusted over a range varying between extremes of the directional sensitivities of the sub-arrays.
The sub-arrays may be arranged in a one-dimensional or two-dimensional array, and that the phase offsets applied to the antenna elements and the sub-array signals provide beam steering respectively in one or two dimensions. The present invention also provides a phased array antenna comprising a matrix of sub-arrays, each sub-array having a corresponding plurality of antenna elements, each antenna element having an associated phase shifting element applying a phase shift to a signal received by the phase shifting element for transmission by, or in response to reception by, a corresponding antenna element, each antenna element of a first sub-array having a first value of a phase shift applied to the corresponding signal, each antenna element of a second sub-array having a second value of a phase shift applied to the corresponding signal.
Each sub-array may further comprise a sub-array summing unit for summing the signals received from each of the antenna elements.
The sub-array summing unit may apply differing weighting values to signals received from differing antenna elements, thereby to provide a degree of aperture shading.
The phased array antenna may further comprise a further summing unit for summing the signals produced by each of the sub-array summing units to produce a received antenna signal having a response equivalent to a combination of the responses of each sub-array.
The further summing unit may apply differing weighting values to signals received from differing sub-arrays, thereby to provide a degree of beam steering.
The summing unit may be arranged to apply a zero weighting to the signals received from certain sub-arrays, thereby eliminating the contribution of the corresponding sub-arrays from the response of the antenna.
Each antenna element of a first sub-array may have a first value of a phase shift applied to the corresponding signal. Each antenna element of a second subarray may haveing a second value (xcfx862) of a phase shift applied to the corresponding signal.
The phased array antenna of the present invention may comprise a plurality of sub-arrays, each sub-array having different phase shifts applied to the signals received from/supplied to their antenna elements. At least two different combinations of sub-array signals are summed to provide an antenna response comprising a corresponding at least two response maxima representing a combination of the individual responses of such sums, each of such maxima representing an antenna response corresponding to a combination of the responses of the sub-arrays whose output is summed. In such a phased array antenna, the resultant antenna response may comprise a plurality of relatively high gain responses in first directions separated by a corresponding plurality of relatively low gain responses in second directions, each of said first directions lying between a pair of second directions. In this case, all of first and second directions preferably lie substantially in a first plane. The first plane may be swept through an angle, itself in a plane substantially orthogonal to the first plane. The antenna may be arranged to perform sweeping by adjustment of phase shifts applied to individual antenna elements, and/or to sub-array signals.
In a phased array antenna according to the present invention, the antenna may comprise a face divided into regions, each region containing the antenna elements of a respective sub-array. The boundaries between the regions may be randomised.
The phased array antenna according to the present invention may further comprise a digital converter associated with each sub-array signal for providing a digital representation of the sub-array signal. The digital representation of the sub-array signals may be provided to at least one digital summing unit to provide a digital representation of the sum of the corresponding sub-array signals.
The direction(s) of maximum response of the antenna of the present invention may be scanned across a certain angle by adjusting the phase shifts applied to the signals from/to each antenna element.
In a method or an antenna according to the present invention, the direction(s) of maximum response of the antenna may be scanned across a certain angle by adjusting the phase shifts applied to the signals from/to each sub-array.